In the technologies of automation and process control, field devices are often applied for the purpose of measuring process variables (sensors) and controlling control variables (actuators).
Field devices for flow, fill level, differential pressure, temperature determination, etc., are well known. For registering the corresponding process variable, such as mass or volume flow rate, fill height, pressure, temperature, etc., the field devices are placed in the immediate vicinity of the relevant process component.
The field devices deliver a measurement signal, representing the measurement of the sampled process variable. This measurement signal is transferred to a central control unit (e.g. a control room or a process control system). As a rule, the entire process control is done from a control unit, where the measurement signals of different field devices are evaluated and, on the basis of the evaluation, control signals for actuators are produced, with the actuators then accordingly influencing the direction of the process.
An example of an actuator is a controllable valve, which regulates the flow of a liquid or gas in a section of a pipeline.
Signal transmission between field device and control unit can occur in digital form over a data bus. Known international standards for such signal transmission are PROFIBUS, FOUNDATION FIELDBUS, and CAN-Bus. In the case of programmable field devices, it is most common to utilize ASIC's (application-specific integrated circuits) or SMD's (surface mounted devices).
Thus, in the case of programmable field devices, ever more “intelligence” is being moved into the field, to the location of actual use.
The corresponding control program of a field device is stored in the field device in non-volatile memory and is executed in a microprocessor, which controls, among other things, the operating, measuring, and control functions of the field device.
Frequently, field devices are supplied with electrical current over the data bus. Electrical current consumption of a field device depends on various factors and is, as a rule, not constant over time. When a measurement is being obtained, an increased supply of current (peak load) becomes necessary. In the times in-between, a lesser supply of current (base load) is sufficient. The electrical current consumption of each field device is set by hardware at a fixed consumption level. Normally, the consumption level corresponds to the peak load, in order to assure that the field device will always have sufficient current for all of its functions. This means that in the times between measurements, unnecessary current is being consumed.
In newer field devices, the consumption level lies between the peak load and the base load. This means, however, that these field devices must contain an energy storer, in order to cover their energy requirement during times when measurement is occurring. Once the energy storer is filled completely, the energy supplied above the base load must be wasted by useless conversion into heat.
Only a certain amount of current (limit level) can be supplied over a data bus. I.e., the number of field devices that can be connected to a data bus is limited. The sum of the consumption levels of the separate field devices must not exceed the limit level. If as many field devices as possible are to be connected to a data bus, then the consumption levels of the separate field devices must be kept as low as possible, and this means, as a consequence, a low measuring rate for the separate field devices.
If the consumption levels of the separate field devices must be lowered, because an additional field device is to be connected to the data bus, this can only be done manually, which is expensive.